Oxidation Basics
Silicon-dioxide is one of the key materials found in microelectronics and the thermal oxidation of silicon is used routinely. Oxides may be used to provide passivation of the silicon surface, to provide isolation between neighboring devices, to mask impurities during diffusion or implantation, and of course to serve as a gate dielectric for MOS devices. The fact that SiO2 is stable, easy to grow and creates an interface with Si with relatively few defects are major factors in the dominance of silicon over other semiconductors in microelectronics.
Silicon dioxide thicknesses between 200 Å and 1 mm can be deposited or thermally grown. When silicon dioxide is formed by deposition, both silicon and oxygen are conveyed to the wafer surface and reacted there. In contrast, thermal oxidation is achieved by reacting a silicon wafer with oxygen or steam at high temperature. Thermally grown oxides generally display superior dielectric properties compared to deposited oxides. The structure of these oxides is amorphous; however, they are strongly bonded to the silicon surface. In the lab process we use only thermal oxides.
Thermal Oxidation Reactions
There are two reactions by which silicon-dioxide may be grown;
Si + O2 –> SiO2, (1)
Si + 2H2O –> SiO2 + 2H2, (2)
Reaction 1 is referred to as “dry” oxidation and reaction 2 as “wet” oxidation. The dry oxidation reaction is slower than the wet oxidation reaction but produces a higher quality, denser oxide.
A model for the oxidation reaction kinetics was proposed over twenty-five years ago and is known today as the Deal-Grove model of thermal oxidation. Calculations based on this model provide a good first-order approximation of the resulting oxide thickness especially for oxide thickness’ greater than 300 angstroms. However the Deal-Grove model is not well suited for oxide films thinner than 300 angstroms. Please consult your textbook and class notes for details of this model.
The oxide is grown by placing wafers on a quartz boat, which is then inserted into a tube furnace at the calculated temperature. The oxidizing gas (O2 or H2O, depending on the type of reaction desired) is fed into the furnace along with an inert carrier gas such as nitrogen. Other gases, such as HCl, may also be added to “getter” impurities (e.g. Na) from the oxide. The lab is presently not equipped to exhaust hazardous gases so HCl is not currently used.
The following section contains procedures used for the wet oxidation of silicon. The dry oxidation of silicon is described in a later section.